7 Energy and Electrons from Glucose The sugar glucose (C 6 H 12 O 6 ) is the most common form of energy molecule. Cells obtain energy from glucose by the

7 Energy and Electrons from Glucose

• The sugar glucose (C6H12O6) is the most common form of energy molecule.

• Cells obtain energy from glucose by the chemical process of oxidation in a series of metabolic pathways.


• Principles governing metabolic pathways:

Metabolic pathways are formed by complex chemical transformations which occur in separate reactions.

Each reaction in the pathway is catalyzed by a specific enzyme.

Metabolic pathways are similar in all organisms.

In eukaryotes, many metabolic pathways are compartmentalized in organelles.

The operation of each metabolic pathway can be regulated by the activities of key enzymes.


• When burned in a flame, glucose releases heat, carbon dioxide, and water.

C6H12O6 + 6 O2 6 CO2 + 6 H2O + energy

• The same equation applies for the biological, metabolic use of glucose.


• About half of the energy from glucose is collected in ATP.

• G for the complete conversion of glucose is –686 kcal/mol.

• The reaction is therefore highly exergonic, and it drives the endergonic formation of ATP.


• Three metabolic processes are used in the breakdown of glucose for energy:

Glycolysis

Cellular respiration

Fermentation


• Glycolysis produces some usable energy and two molecules of a three-carbon sugar called pyruvate.

• Glycolysis begins glucose metabolism in all cells.

• Glycolysis does not require O2; it is an anaerobic metabolic process.


• Cellular respiration uses O2 and occurs in aerobic (oxygen-containing) environments.

• Pyruvate is converted to CO2 and H2O.

• The energy stored in bonds of pyruvate is used to make ATP molecules.


• Fermentation does not involve O2. It is an anaerobic process.

• Pyruvate is converted into: Lactic Acid. Ethanol.

• Breakdown of glucose is incomplete; less energy is released than by cellular respiration.


• Redox reactions transfer the energy of electrons.

• Whenever one material is reduced, another is oxidized.

• During the metabolism of glucose, glucose loses electrons and these are passed along to Oxygen.

Glucose is oxidized. Oxygen is reduced.


• An oxidizing agent accepts an electron or a hydrogen atom.

• A reducing agent donates an electron or a hydrogen atom.


• The coenzyme NAD is an essential electron carrier in cellular redox reactions.

• NAD exists in an oxidized form, NAD+, and a reduced form, NADH + H+.

• The reduction reaction requires an input of energy:

NAD+ + 2H NADH + H+

• The oxidation reaction is exergonic: NADH + H+ + ½ O2 NAD+ + H2O

Table 7.1 Cellular Locations for Energy Pathways in Eukaryotes and Prokaryotes

7 Glycolysis: From Glucose to Pyruvate

• Glycolysis can be divided into two stages:

Energy-investing reactions that use ATP

Energy-harvesting reactions that produce ATP


• The energy-investing reactions of glycolysis:

In separate reactions, two ATP molecules are used to make modifications to glucose.

Phosphates from two ATPs are added to the carbon 6 and carbon 1 of the glucose molecule to form fructose 1,6-bisphosphate.

The enzyme aldolase splits the molecule into two 3-C molecules that become glyceraldehyde 3-phosphate (G3P).

Figure 7.6 Glycolysis Converts Glucose to Pyruvate (Part1)

Figure 7.6 Glycolysis Converts Glucose to Pyruvate (Part2)


• The energy-harvesting reactions of glycolysis: The first reaction (an oxidation) releases free

energy that is used to make two molecules of NADH + H+, one for each of the two G3P molecules.

Two other reactions each yield one ATP per G3P molecule. This part of the pathway is called substrate-level phosphorylation.

The final product is two 3-carbon molecules of pyruvate.

Figure 7.7 Changes in Free Energy During Glycolysis

7 Pyruvate Oxidation

• Pyruvate is oxidized to acetate which is converted to acetyl CoA.

• Pyruvate oxidation is a multistep reaction catalyzed by an enzyme complex attached to the inner mitochondrial membrane.

• The acetyl group is added to coenzyme A to form acetyl CoA. One NADH + H+ is generated during this reaction.

Figure 7.8 Pyruvate Oxidation and the Citric Acid Cycle (Part 1)

7 The Citric Acid Cycle

• The citric acid cycle begins when the two carbons from the acetate are added to oxaloacetate, a 4-C molecule, to generate citrate, a 6-C molecule.

• A series of reactions oxidize two carbons from the citrate. With molecular rearrangements, oxaloacetate is formed, which can be used for the next cycle.

• For each turn of the cycle, three molecules of NADH + H+, one molecule of ATP, one molecule of FADH2, and two molecules of CO2 are generated.

Figure 7.8 Pyruvate Oxidation and the Citric Acid Cycle (Part 2)

7 The Respiratory Chain:Electrons, Protons, and ATP Production

• The respiratory chain uses the reducing agents generated by pyruvate oxidation and the citric acid cycle.

• The flow of electrons in a series of redox reactions causes the active transport of protons across the inner mitochondrial membrane, creating a proton concentration gradient.

• The protons then diffuse through proton channels down the concentration and electrical gradient back into the matrix of the mitochondria, creating ATP in the process.

• ATP synthesis by electron transport is called oxidative phosphorylation.


• As electrons pass through the respiratory chain, protons are pumped by active transport into the intermembrane space against their concentration gradient.

• This transport results in a difference in electric charge across the membrane.

• The potential energy generated is called the proton-motive force.


• Chemiosmosis is the coupling of the proton-motive force and ATP synthesis.

• NADH + H+ or FADH2 yield energy upon oxidation.

• The energy is used to pump protons into the intermembrane space, contributing to the proton-motive force.

• The potential energy from the proton-motive force is harnessed by ATP synthase to synthesize ATP from ADP.

Figure 7.12 A Chemiosmotic Mechanism Produces ATP (Part 1)

Figure 7.12 A Chemiosmotic Mechanism Produces ATP (Part 2)


• Synthesis of ATP from ADP is reversible.

• The synthesized ATP is transported out of the mitochondrial matrix as quickly as it is made.

• The proton gradient is maintained by the pumping of the electron transport chain.

7

Fermentation

7 Fermentation: ATP from Glucose, without O2

• When there is an insufficient supply of O2, a cell cannot reoxidize cytochrome c.

• This continues until the entire respiratory chain is reduced.

• NAD+ and FAD are not generated from their reduced form.

• Pyruvate oxidation stops, due to a lack of NAD+.

• Likewise, the citric acid cycle stops, and if the cell has no other way to obtain energy, it dies.


• Some cells under anaerobic conditions continue glycolysis and produce a limited amount of ATP if fermentation regenerates the NAD+ to keep glycolysis going.

• Fermentation uses NADH + H+ to reduce pyruvate, and consequently NAD+ is regenerated.


• Some organisms are confined to anaerobic environments and use only fermentation.

These organisms lack the molecular machinery for oxidative phosphorylation.

They also lack enzymes to detoxify the toxic by-products of O2, such as H2O2.


• In lactic acid fermentation, an enzyme, lactate dehydrogenase, uses the reducing power of NADH + H+ to convert pyruvate into lactate.

• NAD+ is replenished in the process.

• Lactic acid fermentation occurs in some microorganisms and in muscle cells when they are starved for oxygen.

Figure 7.14 Lactic Acid Fermentation


• Alcoholic fermentation involves the use of two enzymes to metabolize pyruvate.

• First CO2 is removed from pyruvate, producing acetaldehyde.

• Then acetaldehyde is reduced by NADH + H+, producing NAD+ and ethanol.

Figure 7.15 Alcoholic Fermentation

7 Contrasting Energy Yields

• A total of 36 ATP molecules can be generated from each glucose molecule in glycolysis and cellular respiration.

• Fermentation has a net yield of 2 ATP molecules from each glucose molecule.

• The end products of fermentation contain much more unused energy than the end products of aerobic respiration.

7 Relationships between Metabolic Pathways

• Glucose utilization pathways can yield more than just energy. They are interchanges for diverse biochemical traffic.

• Intermediate chemicals are generated that are substrates for the synthesis of lipids, amino acids, nucleic acids, and other biological molecules.

Figure 7.17 Relationships Among the Major Metabolic Pathways of the Cell


• Catabolic interconversions: Polysaccharides are hydrolyzed into glucose.

Lipids are converted to fatty acids, which become acetate (then acetyl CoA), and glycerol, which is converted to an intermediate in glycolysis.

Proteins are hydrolyzed into amino acids, which feed into glycolysis or the citric acid cycle.


• Anabolic interconversions: Gluconeogenesis is the process by which

intermediates of glycolysis and the citric acid cycle are used to form glucose.

Intermediates can form amino acids.

The citric acid cycle intermediate -ketoglutarate is the starting point for the synthesis of purines. Oxaloacetate is a starting point for pyrimidines.


• What happens if inadequate food molecules are available?

Glycogen stores in muscle and liver are used first.

Fats are used next, then proteins.

7 Regulating Energy Pathways

• The amount and balance of products a cell has is regulated by allosteric control of enzyme activities.

• Control points use both positive and negative feedback mechanisms.

• The main control point in glycolysis is the enzyme phosphofructokinase.

• This enzyme is inhibited by ATP and activated by ADP and AMP.

7 Regulating Energy Pathways

• The main control point of the citric acid cycle is the enzyme isocitrate dehydrogenase which converts isocitrate to a-ketoglutarate.

• NADH + H+ and ATP are inhibitors of this enzyme. NAD+ and ADP are activators of it.

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7 Energy and Electrons from Glucose The sugar glucose (C 6 H 12 O 6 ) is the most common form of energy molecule. Cells obtain energy from glucose by the